Have you ever heard that a circuit board is “bendable?” Yes, it is possible for a circuit board to bend and the name for it is flexible copper PCB.
In case this is your first time of hearing the term, this article is a pointer to the right direction. You will find out the meaning of flexible copper circuit board, as well as some of the properties.
What is Flexible Copper PCB?
As the name suggests, it is a type of Printed Circuit Board (PCB) that is flexible, i.e., can be bent or turned into different forms – for different purposes.
Understanding the Structure of Flexible Copper PCB
The first step to understanding how a flexible circuit board works is by taking note of the structure.
One unique feature is that it takes a similar pattern or shape to the rigid Printed Circuit Board (PCB), in the sense that the board has a good dielectric material and higher temperature resistances.
With that being said, here is a breakdown of all that go into the structure of the flexible copper circuit board.
a. The Dielectric Substrate Film
This is the first thing you will notice about the flexible copper PCB. The board uses a layer of Polyimide (Pi), which serves as the base material. The material is also characterized by high resistance to both the temperature and traction.
b. Electrical Conductors
This is where the “copper” in flexible copper PCB is derived from. The electrical conductors refer to the copper, which represents and is used for routing the traces of the circuit board.
c. The Coating/Protective Finish
This is the “protective feature” used to prevent a contamination of the copper in particular and the PCB in general.
The protective finish is commonly made of cover coat.
d. The Adhesive Material
Next on the list is the adhesive material, which is the material required for making the “board stick.”
The adhesive layer of epoxy resin is commonly used but you can also substitute with polyethylene. The goal of the adhesive material is to join the several parts of the flexible copper PCB, especially the bonding of the copper foil layer on the top layer.
The above is the function of the adhesive material on the single-sided flexible PCB. For the double-sided or multilayered version of the circuit board, you can use the adhesive material to join the upper and bottom layers of the copper foil.
The Classifications of Flexible Copper PCB
You can find different types of flexible circuit boards in the market. While the single-sided variant is the one you can commonly interact with, it has other variants.
There is the double-sided flexible board, which, as the name suggests, has two sides.
Then, there is the multi-sided or multilayered flexible copper board, which uses more than 2 layers or sides.
The Fabrication Processes of Flexible Copper PCBs
Note that the process of fabricating or manufacturing the flexible circuits differ, by the number of sides or layers. For example, you will find the process of the single-sided variant different and possibly, easier than that of the double-sided or the multilayered.
a. Fabricating a Single-Sided Flexible Circuit Board
The flexible board using a single side is fabricated, first, by making a slit cut of the roll-shaped Polyimide copper flexible base material into the size of the flexible PCB, as specified by the customer(s).
The next steps include the development and transfer of the circuit to the copper foil. The process of exposure etching is used for the purpose and while the etching part of the PCB is retained, the other (residual) parts are taken off.
The required holes are made on the PCB, before E-testers will be used to validate the circuit board’s performance.
Subsequent steps to take include:
- Pasting a Polyimide protective film on the copper layer to protect it from oxidation and contamination;
- Lamination;
- Apply the solder mask and legends;
- Using curing bake to cure the solder mask and legends;
- Implementing the surface treatments, such as Immersion Gold, OSP, and LF-HASL;
- Doing a second test to confirm that the circuit board is of high-quality;
- Fixing-up the board’s appearance, before packaging it.
- The last round of inspections is made to validate the package before it is shipped.
b. Fabricating Double-Sided and Multilayered Flexible Copper PCBs
The processes of manufacturing double-sided and multi-sided flexible circuit boards are similar to those of the single-sided variants – but with a few additions.
For example, as soon as the base material is transferred to the flexible PCB size, the first drilling should be done. This is to be followed by the electroplating the copper foil – and then, the plating of the vias with copper.
The circuit design will be transferred to the copper foil via exposure, and etching process takes place – removing the residual parts and retaining the etching part.
The fabrication continues with the punching of holes on the PCB, running the first round of inspections with E-testers and doing the cleaning and grinding of the board.
The protection of the PCB is done with the pasting of the Polyimide protective film, application of the solder mask and legends; curing of the legends and the solder mask; and the implementation of surface treatment as a way of preventing pad oxidation.
Just like the single-sided flexible copper PCB, the next processes include:
- Running a second round of test/inspection to confirm that the circuit board is in good working condition.
- Using a knife mold to cut-off the size, as specified by the customer(s).
- Taking note of the physical appearance of the board and fixing any design flaws at this stage.
- Packaging the double/multi-sided flexible copper PCBs and having random inspections to ensure the consistency of the number of labels, boxes and marks before shipping.
Final Words
Flexible copper circuit boards are popular because of the reliability, freedom it grants the designer to mold it into different shapes and for different purposes. These boards also have higher temperature resistances (especially between -200˚C and 400˚C) – and they can help reduce the incidences of human errors during wiring.
